1. Field of the Invention
The present invention relates to power tools in which a rotational speed control of a motor is performed by adjusting an AC power, which may be supplied to the motor via a power cord having a plug connectable to an electrical outlet.
2. Description of the Related Art
A known technique of controlling a rotational speed of a motor connectable to an AC power source having a commercial frequency is disclosed in Japanese Laid-Open Patent Publication No. 8-154392. A rotational speed control device for a motor of this publication is shown in FIG. 5 and includes a control section, a correction section and a signal converting section. The rotational speed is controlled by the operation of a switching element, so that potential deviation of a rotational speed from a set value can be decreased. The rotational speed can be detected by a rotational speed detecting device. The set value can be set by a rotational speed setting device. More specifically, the rotational speed control device can vary the on-operation timing of the switching element depending on the phase of the AC power source voltage, so that the period of time for supplying the power to the motor can be adjusted for controlling the rotational speed of the motor. This control technique is known as a phase control technique and has been generally applied to control circuits for power tools for use with AC power sources.
Power tools for use with AC power sources generally have a power source cord with a plug and are used by inserting the plugs into electrical outlets. Therefore, the configurations of the plugs are determined to correspond to the configurations of the electrical outlets. For this reason, normally, the plugs cannot be used for connection with outlets of DC power sources. However, some welding machines having engines and are distributed in the countries other than Japan that have DC outlets that are similar to normal AC outlets. With this configuration it is possible that the plugs of power tools for use with AC power sources are wrongly connected to such DC outlets.
In the case that the plugs of power tools for use with AC power sources and having the aforementioned rotational speed control devices for motors are connected to DC outlets, the switching elements may not be switched off once they have been switched on. Therefore, the motors may continuously receive maximum power. As a result, the motors are driven at a full rotational speed and therefore without any speed control of the motor.
Therefore, there is a need in the art for power tools that have a function of distinguishing a power source between an AC power source and a DC power source.